Abstract
Imaging with THz radiation has proved an important tool for both fundamental science and industrial use. Here we review a class of THz imaging implementations, named coherent lensless imaging, that reconstruct the coherent response of arbitrary samples with a minimized experimental setup based only on a coherent source and a camera. After discussing the appropriate sources and detectors to perform them, we detail the fundamental principles and implementations of THz digital holography and phase retrieval. These techniques owe a lot to imaging with different wavelengths, yet innovative concepts are also being developed in the THz range and are ready to be applied in other spectral ranges. This makes our review useful for both the THz and imaging communities, and we hope it will foster their interaction.
Highlights
Terahertz (THz) radiation denotes the portion of the electromagnetic spectrum whose frequency ranges from 0.1 THz to 10 THz, lying between the infrared and microwave regions [1]
Despite most coherent lensless imaging techniques using THz radiation bear no difference from the corresponding implementations at shorter wavelengths, e.g., with visible light, x rays, or electrons, the tremendous increase in wavelength has a relevant impact on the design and analysis of a THz imaging experiment
Like all conventional imaging techniques, all the coherent lensless imaging techniques presented in this contribution are subject to the diffraction limit, i.e., the lateral resolution ρlat depends on the wavelength and the numerical aperture (NA) according to ρlat = λ/(2 NA)
Summary
Terahertz (THz) radiation denotes the portion of the electromagnetic spectrum whose frequency ranges from 0.1 THz to 10 THz, lying between the infrared and microwave regions [1]. Ultrafast lasers enabled the development of broadband THz sources and coherent detectors [14] Along these lines, imaging with THz time-domain spectroscopy (THz-TDS) [15] has found widespread use in both industrial [16] and scientific applications [17,18]. The most important advantage of imaging with THz-TDS is its ultrafast detection, which allows direct measurement of the time evolution of the THz electric field, so that the complex refractive index of the object can be extracted [21] This comes at the expense of the flexibility of the setup, which should always feature synchronization between source and detector.
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